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Tue, 31 Mar 2015 17:56:50 +0000en-UShourly1http://wordpress.org/?v=3.9.1Synthesizing Graphene with Chemical Vapor Depositionhttp://www.comsol.com/blogs/synthesizing-graphene-chemical-vapor-deposition/
http://www.comsol.com/blogs/synthesizing-graphene-chemical-vapor-deposition/#commentsThu, 06 Nov 2014 19:48:45 +0000http://com.staging.comsol.com/blogs/?p=39451With its growing use in numerous applications, the demand for graphene has steadily increased over the years. This heightened interest has prompted new research behind the methods for synthesizing graphene — one of which is chemical vapor deposition. See how one research team used modeling to analyze and enhance the CVD graphene growth mechanism.

Meeting the Demand for Graphene

You’ve probably heard the word “graphene” in the news and here on the blog numerous times, usually with references to its powerful capabilities in advancing technology within various industries. It’s not every day that a material quite as unique and powerful as graphene comes along and it’s safe to say the world has taken notice.

Graphene has been a relevant topic on the minds of many, ourselves included. In a recent series of blog posts, we highlighted the revolution behind this material, from its exotic properties and production methods to simulating its use in various applications.

Our last post in the series emphasized research on the “wonder material” that led to the accidental discovery of 2D glass. While the discovery in itself is remarkable, the point on which I’d like to focus is how they actually grew the graphene used in the research — through chemical vapor deposition (CVD).

The Science Behind Chemical Vapor Deposition

Chemical vapor deposition describes the chemical process designed to create solid materials that perform strongly and are highly pure. In this method, gas molecules are combined in a reaction chamber containing a heated substrate. The interaction between the gases and the heated substrate causes the gases to react and/or decompose on the substrate’s surface, thus producing a material film.

This synthesis method is particularly valued for its ability to produce materials that are rather high in quality. Compared to other coating methods, the resulting materials in chemical vapor deposition tend to possess greater purity, hardness, and resistance to agitation or damage. An additional advantage within this method is the wide range of materials that can be deposited, one of which is graphene.

Graphene Synthesis

Among synthesis techniques, chemical vapor deposition has proved promising in the development of high-quality graphene films. The process involves growing graphene films on different kinds of substrate that utilize transition metals. One such example is nickel (Ni). This involves the diffusion of decomposed carbon atoms into nickel at a high temperature, followed by the precipitation of carbon atoms on the surface of the nickel during the cooling process.

Because of the multiplicity of the growth conditions in the CVD method, producing a single-layer graphene and maintaining control over the quality of the graphene film can be very challenging. One research team from the University of Arkansas recognized the need to better understand the growth mechanism as well as optimal conditions for graphene production.

Understanding the CVD Graphene Growth Mechanism

Using COMSOL Multiphysics, the researchers created a graphene synthesis model to analyze the dissolution-precipitation mechanism for CVD graphene growth on nickel. In the study, they analyzed factors affecting the number of graphene layers synthesized, including growth time and temperature, rate of cooling, carbon solubility in nickel, and the nickel’s film thickness.

In analyzing the diffusion of the carbon atoms, the team found that the greater the temperature within the Ni film, the more accelerated the diffusion process was. From their results, they also concluded that additional time was needed for carbon atoms to reach their saturated state in thicker Ni film.

Additionally, the researchers modeled supersaturation by cooling. In the supersaturation process, carbon atoms become segregated on the surface of the Ni thin film. When cooling the film from 900°C to 725°C, 1.7 layers of graphene were obtained on the film’s surface. This resulting number of graphene layers proved reasonable in comparison to experimental data.

Download the Research

]]>http://www.comsol.com/blogs/synthesizing-graphene-chemical-vapor-deposition/feed/02D Materials, It’s Not Just About Graphenehttp://www.comsol.com/blogs/2d-materials-not-just-about-graphene/
http://www.comsol.com/blogs/2d-materials-not-just-about-graphene/#commentsFri, 13 Dec 2013 13:27:33 +0000http://com.dev.comsol.com/blogs/?p=23781You’ve heard the story: a couple of scientists discovered graphene when they repeatedly pulled a strip of adhesive tape off a layer of graphite. Graphene has been all the rage due to its incredible strength, low weight, and electronic properties, but it’s not the only material of its kind. There are plenty of other 2D materials to consider for electrical applications — some of which may work together with graphene, and others that can be used in its place.

Natural Graphene

Graphene has a slew of benefits and unusual material properties. To use the same comparisons as my colleague Dan did in his previous blog post, graphene is about 200 times stronger than steel and has a higher electrical and thermal conductivity than copper. Dan attributes the high electrical and thermal conductivity to its high room temperature electron mobility of greater than 15,000 cm2/(V-s), and up to 200,000 cm2/(V-s) in certain forms. An IEEE Spectrum article from earlier in the year put that last part in simple terms:

“At low temperature, for example, electrons can zip through the material 100 times as fast as they do in silicon for a given voltage.”

Additionally, like I mentioned earlier, the material also weighs very little — less than 1 milligram per square meter, in fact. All of this makes natural graphene a great option for creating fast RF transistors and other electronics.

Having said all that, graphene in its natural state is not entirely without limitations. As we can see in the IEEE Spectrum article, natural graphene does not have a bandgap (or a threshold for electronic excitations) like semiconducting materials do, meaning you cannot turn the current flow off. As you may already know, being able to make switches in currents is important for making logic, which is why semiconducting silicon has been so popular for modern electronics.

What about the research that’s on track to introduce bandgaps to graphene? We might not quite be there yet, according to the article I mentioned. A bandgap can be introduced by stacking layers of the material on top of one another or by cutting it into nanoribbons, although they say this will typically bring forth an important side-effect: reduced electron speed.

Other 2D Materials

While graphene research and manufacturing is well underway and widely talked about, there are other 2D materials out there that have not been explored as thoroughly yet but do show promise, on their own or in combination with graphene.

Boron Nitride

Take boron nitride, for example. Whereas graphene has no bandgap, boron nitride has a very wide bandgap — the same as diamonds. The bandgap is so large that it is not appropriate for switches, but instead it makes for a good insulator. Advancements have been made in terms of combining this material with graphene to create incredibly thin circuits.

Boron nitride.

Silicene

A competitor to graphene, silicene is a one-atom thick version of silicon that can be grown on silver surfaces. With some similar material properties to graphene (like massless Dirac fermions, for instance), silicene takes it one step further with lower group symmetry and stronger spin-orbit coupling, perhaps making it better suited for integrating with silicon-based electronics than graphene.

Stanene

Using simulation tools, researchers recently combined 2D monolayers of tin with fluorine atoms to produce what they call stanene, a material that appears to be very similar to graphene. The researchers project that stanene will be able to conduct energy at 100% efficiency along its edges at room temperature, while its center behaves like an insulator. Four lanes of zero-resistance data traffic is supposedly supported, two lanes going in opposite directions along each edge.

In other words, the material is thought to act like dual, side-by-side superconducting wires. There is a fascinating difference between this new material and a normal conductor in terms of resistance. Whereas with a normal conductor, a longer wire would provide a larger resistance, but when it comes to stanene it’s all about where the end-points are in contact with the chip circuitry, resulting in a resistance that remains constant regardless of length.

The researchers are still awaiting experimental confirmation on their new material, but if it is indeed confirmed, stanene could be the next big thing since graphene.

Graphene Research Leads to Accidental Discovery of 2D Glass

The graphene journey has come a long way since that fateful day in 2004. According to gizmag.com, graphene research spawned the accidental discovery of the world’s thinnest glass in 2012. Measuring only two atoms thick, it’s a truly remarkable discovery.

The 2D glass came about when two Cornell researchers grew graphene (via chemical vapor deposition) on a copper film attached to a quartz substrate. When the researchers noted that the graphene layer was discolored, they decided to test it. That’s when they realized that the discolored portion was actually silica glass — in a one-molecule layer. Although they have not yet been able to reproduce these results, the researchers speculate that this type of glass could potentially be used in nanotechnology.

Further Reading

http://www.comsol.com/blogs/2d-materials-not-just-about-graphene/feed/0The Graphene Revolution: Part 5http://www.comsol.com/blogs/the-graphene-revolution-part-5/
http://www.comsol.com/blogs/the-graphene-revolution-part-5/#commentsWed, 08 May 2013 17:01:39 +0000http://com.dev.comsol.com/blogs/?p=12557In a paper titled “Choosing a Gate Dielectric for Graphene Based Transistors“, the applications of a semiconducting form of graphene are examined. As we have seen before, single-layer graphene is not a semiconductor, it is a zero bandgap conductor (a semimetal). Efforts are well underway to introduce bandgaps to graphene, which would make it semiconducting with a room temperature mobility an order of magnitude higher than silicon. The race is already underway to find applications for such a material once the remaining technical challenges have been overcome. An application of semiconducting graphene is the design of next-generation, fast switching semiconductor metal oxide field effect transistors (MOSFETs).

What is a MOSFET?

The basic idea of a MOSFET is to apply a gate voltage to control the drain-to-source resistance and thus the drain current (see image below). At a certain gate-to-source voltage (VGS), and at low drain-to-source voltages (VDS), the drain current is almost linearly dependent on VDS. When VDS increases, the drain current saturates. The level of saturation depends on the gate-to-source voltage and the switching time depends on the mobility of the semiconductor. The higher the mobility of the semiconducting material, the faster the current can be switched on and off.

Semiconductor Physics

Semiconductor physics is extremely complicated. Strictly speaking, the Boltzmann equation should be solved with Maxwell’s equations in order to describe the device physics completely. Since this is computationally intractable, the most common approach for modeling semiconductors is to solve a set of drift diffusion equations coupled to Poisson’s equation:

Here n is the number density of electrons, p is the number density of holes, V is the electrostatic potential, Rn is the electron recombination rate, Rp is the hole recombination rate, Jn is the electron current, Jp is the hole current. Solving this set of equations allows you to construct the voltage-current characteristics of semiconducting devices. The semiconductor equations are highly non-linear and require special numerical methods to solve.

Our Brand New Semiconductor Module

As we recently announced, a dedicated product for modeling semiconductor devices is now available within the COMSOL platform. The Semiconductor Module, as it’s called, allows for detailed analysis of semiconductor device operation at the fundamental physics level. The module is based on the drift-diffusion equations with isothermal or non-isothermal transport models. Two numerical methods are provided: the finite volume method with Scharfetter-Gummel upwinding and a Galerkin least-squares stabilized finite element method. The module provides an easy-to-use interface for analyzing and designing semiconductor devices, greatly simplifying the task of device simulation on the COMSOL platform.

Models for semiconducting and insulating materials in addition to boundary conditions for ohmic contacts, Schottky contacts, and gates are provided as dedicated features within the Semiconductor Module. The module includes enhanced functionality for modeling electrostatics. System level and mixed device simulations are enabled through an interface for electrical circuits with SPICE import capability.

The Semiconductor Module is useful for simulating a range of practical devices. The built-in Model Library contains a suite of models designed to provide straightforward instruction and demonstrate how to use the interface to simulate your own devices. The Semiconductor Module is particularly relevant for simulating transistors including bipolar, metal semiconductor field-effect transistors (MESFETs), metal-oxide-semiconductor field-effect transistors (MOSFETs), Schottky diodes, thyristors, and P-N junctions.

The Semiconductor Module could hence be used to investigate the device characteristics of graphene-based semiconductors similar to the ones explained in the paper referenced at the beginning of this blog post.

This Concludes the Graphene Series…

Over the past few months, in the previous four posts of our blog series, we have read lots about the history, applications, and manufacture of graphene. I’ve enjoyed talking about this topic very much — but I’ve only scratched the surface. There are still many sub-topics that haven’t been explored, and I encourage you to keep up with the latest graphene-related developments in technical magazines and publications. Whether you’re interested in the applications or the manufacture of graphene, COMSOL offers a wide range of products that can provide insight and a better understanding of these processes.

Further Reading

]]>http://www.comsol.com/blogs/the-graphene-revolution-part-5/feed/1The Graphene Revolution: Part 4http://www.comsol.com/blogs/the-graphene-revolution-part-4/
http://www.comsol.com/blogs/the-graphene-revolution-part-4/#commentsThu, 02 May 2013 19:33:46 +0000http://com.dev.comsol.com/blogs/?p=12429Graphene can be created by way of thermal decomposition at high vacuum. In order to design and optimize these high vacuum systems engineers might look to simulation, but there are currently not many modeling tools that are up to the task. Let’s have a look at how vacuum systems are relevant to graphene production, why you should simulate them, and how.

Obstacles in Bringing Graphene to the Masses

A while back I blogged about graphene and some of its properties, as well as some of the modeling tools available in COMSOL for investigating applications of this exotic material. One of the main hurdles to overcome before graphene can be released to the mass market is how to manufacture it in an economical way. Many production methods are currently employed, including exfoliation, expitaxial growth on a suitable substrate, reduction of graphene oxide, pyrolisis or growth from metal-carbon melts.

In this thesis titled “Growth of Graphene Films on Pt(111) by Thermal Decomposition of Propylene” the author explains how graphene can be manufactured using thermal decomposition at high vacuum. As my colleague, Bjorn, mentions in his blog post on “What is Molecular Flow?” there are few modeling tools available to assist with design and optimization of high vacuum systems, especially if the system is non-isothermal. Gases at low pressures cannot be modeled with conventional fluid dynamics tools because kinetic effects become important as the mean free path of the gas molecules becomes comparable to the length scale of the flow. The pressure on surfaces depends primarily on the line of sight with respect to molecular sources and sinks in the vacuum system. The non-isothermal vacuum system described in the thesis would inevitably be expensive to build, so any design optimizations that could be made before construction would result in big savings further down the line. Some estimations on the fluxes onto surfaces and deposition rates are given in the thesis, but these don’t take into account the geometry of the system, the fact that different surfaces have different temperatures, or the location of the pumps.

Soon You Can Model High Vacuum Systems

We at COMSOL are getting ready to announce that a dedicated product for modeling high vacuum systems, the Molecular Flow Module, will be made available to our customers in early May. The product will greatly expand on the capabilities currently available in the Microfluidics Module. The Molecular Flow Module is a collection of tailored physics interfaces for the simulation of kinetic gas flows. The module includes two physics interfaces: the Transitional Flow interface and the Free Molecular Flow interface. The Transitional Flow interface uses a discrete velocity/lattice Boltzmann approach to solve low velocity gas flows in the transitional flow regime. The Free Molecular Flow interface uses the angular coefficient method to compute the particle flux, pressure, and heat flux on surfaces. The number density can be computed on domains, surfaces, edges, and points and is thus available to be coupled to other physics.

Why Simulate Vacuum Systems?

Simulations can increase the pace of product development and also lead to a better understanding of vacuum systems for design engineers. High and ultra-high vacuum systems are typically very expensive to construct and test. It is not only the cost to design and machine parts; a large amount of time can be spent baking out, pumping down, and checking for leaks in the system. Imagine if a smaller pump could be fitted onto a process tool simply by placing the pump port at a specific location. Or that the chamber met the necessary specification on the first build. The cost savings would be substantial. The Molecular Flow Module will allow you to test and optimize different designs before you have to start cutting metal.

Applications Relevant to Vacuum Processing and Graphene Production

In order to accurately model non-isothermal molecular flows and deposition rates onto substrates on arbitrarily complicated geometries, a sophisticated modeling approach is required. Since gas molecules interact with surfaces more frequently than they interact with one another, the flow of gas is determined by collisions with the surfaces in the system. It becomes necessary to solve a complicated integral equation in order to compute the molecular flux, pressure, heat flux, and number density in the system. The molecular flux can be used in combination with a suitable differential equation to determine the deposition rate and the deposited film thickness as well.

Let’s have a look at a couple of modeling examples.

Example 1: Compute the thickness of a thermally evaporated gold film

Gold is evaporated from a thermal source at a temperature of 2000 K onto a substrate held on a fixed surface. The deposited film thickness on the substrate and the chamber walls is computed.

Molecular flow calculations can show uniformity and deposition rate onto a wafer from a thermally evaporated material.

Example 2: Adsorption and Desorption of Water in a Load Lock Vacuum System

The system consists of two chambers separated by a gate valve (not shown in the plot below). The lower cylindrical chamber is the load lock chamber. The upper spherical chamber is a high vacuum chamber, which is not vented during the sample loading process. The vacuum pump is located opposite the gate valve, and has a constant speed of 500 l/s.

Time dependent adsorption and desorption of water in a vacuum system at low pressures. The water is introduced into the system when a gate valve to a load lock is opened and the subsequent migration and pumping of the water is modeled.

Further Reading

]]>http://www.comsol.com/blogs/the-graphene-revolution-part-4/feed/0The Graphene Revolution: Part 3http://www.comsol.com/blogs/the-graphene-revolution-part-3/
http://www.comsol.com/blogs/the-graphene-revolution-part-3/#commentsWed, 10 Apr 2013 12:02:44 +0000http://com.dev.comsol.com/blogs/?p=11963Everyone’s talking about graphene right now. When was the last time a material received this much attention? Sure, other materials have peaked our interest before, but when something breaks into more mainstream news you know it’s going to be a very big deal.

Graphene Takes Media by Storm

By now you’ve probably also seen my colleague Dan’s two prior blog posts on the “Graphene Revolution”. In Part 1 he introduced us to the material itself, and in Part 2 he showed three interesting simulation scenarios involving the material du jour. Yet, as any quick Google search will reveal, we’re not the only ones talking about this.

Business Insider recently published an article postulating that patent wars over graphene have already begun. The author of the article also reiterates just how special of a material graphene is; it’s incredibly thin while also mind-blowingly strong. Graphene further proves useful for a great many applications, giving way to this ongoing “patent war”.

Digital financial media company TheStreet ran a story boasting graphene as the (latest…) renewable energy solution. The article suggests that graphene can be turned into a capacitor that can help power electric cars for longer distances than are currently possible. Not only that, apparently this type of capacitor would also be able to recharge in very little time. The punchline of this particular story is that the science here is done and we just need to get going on engineering and mass-producing products using the material. The one obstacle we need to get around before this can happen is, of course, the patent wars highlighted by Business Insider.

The high-prized strength of this material is usually the central theme of articles found on the subject, but one group of researchers point out that “even graphene has weak spots“. In particular, the researchers studied graphene sheets that had been generated in a lab. Apparently these sheets are seldom perfect arrays of hexagons, resulting in graphene domains that are not quite lined up with each other. Instead, a seven-atom ring occurs at the grain boundary junctions, thus creating weak spots in the material. The scientists discovered that where these weak spots appear, polycrystalline graphene has about half the strength of pristine samples under tension. Uh-oh. These findings directly contradict older research stating that graphene’s strength in fact lies in its defects. This group of researchers does suggest that the key here is “the angles at which the individual sheets are stitched together”, and that in some cases the grain boundaries can be just as strong as pure graphene.

The Graphene Revolution Continues

So who’s right? I’m inclined to say we cannot ignore the weak spots, but I also don’t think they will put an end to the Graphene Revolution. It’s already too strongly underway. With more research, scientists and engineers are bound to uncover more potential uses for this material, and get an even better understanding of how it works — in isolation and together with other materials and physics.

We will of course continue to cover this topic, so stop back soon for Part 4 of our graphene blog series.

Further Reading

]]>http://www.comsol.com/blogs/the-graphene-revolution-part-3/feed/0The Graphene Revolution: Part 2http://www.comsol.com/blogs/the-graphene-revolution-part-2/
http://www.comsol.com/blogs/the-graphene-revolution-part-2/#commentsWed, 27 Mar 2013 12:58:08 +0000http://com.dev.comsol.com/blogs/?p=11529In a previous blog entry I discussed some of the exotic properties of graphene. The fact that graphene consists of a single layer of atoms means the aspect ratio of any graphene-based structure may be very high. High aspect geometries present their own array of modeling challenges.

Thermal Modeling of Graphene

COMSOL provides a wide range of tools to assist in modeling geometries and features with very high aspect ratios. Thermal modeling of graphene quilts was recently performed in COMSOL and appeared in Nature Communications: “Graphene quilts for thermal management of high-power GaN transistors“. The authors of the paper use COMSOL Multiphysics to show that the local thermal management of a AlGaN/GaN heterojunction field effect transistor (HFET) can be substantially improved via introduction of the additional heat-escaping channels — top-surface heat spreaders — made of few-layer graphene (FLG).

The Heat Transfer interface in COMSOL Multiphysics allows you to model very high aspect ratio components using the Highly Conductive Layer feature. This feature solves the heat transfer equation only in the tangential plane of the surface, thus removing the need to use a very fine mesh on the high aspect ratio layers. Computation time and memory usage is substantially reduced using this approach.

The settings window for the Highly Conductive Layer feature.

Electrical Modeling of Graphene

COMSOL has been used to study the electrical characteristics of graphene from as far back as 2006. In this paper researchers used COMSOL to infer the in-plane and transverse electrical conductivity of a graphene-based composite material. Providing tensor quantities for electrical conductivity is very easy in COMSOL Multiphysics. You simply need to provide the elements of the conductivity tensor, which can be functions of temperature or any other quantity.

In the settings windows for the Current Conservation feature in the Electric Currents interface, it is easy to specify anisotropic electrical conductivity.

Structural Modeling of Graphene

COMSOL may also be used to model structural applications of graphene. In this paper researchers calculated the deflection and strain induced in a graphene membrane subjected to a pressure difference. Changes in the band structure could be detected electrically, suggesting a potential application as ultra-sensitive pressure sensor. The Shell interface, available in the Structural Mechanics Module, is intended for the structural analysis of thin-walled structures and thus perfect for such applications. The formulation used in the Shell interface is a Mindlin-Reissner type, which means that transverse shear deformations are accounted for. This means that highly accurate results can be obtained without the need to mesh a very thin structure.

Settings window for the material model in the Shell interface.

Relevant Example Models

Now that we’ve gone through thermal, electrical, and structural modeling concepts you might wonder if there is one grand example that demonstrates all of the concepts at once. There is — it’s located in the Model Library and shown below as well.

This multiphysics example simulates the electrical heat generation, heat transfer, and mechanical stresses and deformations of a heating circuit device. The model uses the Heat Transfer interface of the Heat Transfer Module in combination with the Shell and Conductive Media DC interfaces from the AC/DC Module, and the Solid, Stress-Strain, and Shell interfaces from the Structural Mechanics Module.

]]>http://www.comsol.com/blogs/the-graphene-revolution-part-2/feed/0The Graphene Revolution: Part 1http://www.comsol.com/blogs/the-graphene-revolution-part-1/
http://www.comsol.com/blogs/the-graphene-revolution-part-1/#commentsTue, 12 Mar 2013 12:18:36 +0000http://com.dev.comsol.com/blogs/?p=11003Graphene is a special type of material consisting of a single layer of carbon atoms arranged in a hexagonal lattice. Graphene in its stable form was discovered at the University of Manchester in 2003 (coincidentally while I was there studying for my Masters degree) and resulted in Nobel Prizes in 2010 for the two researchers who discovered it. Recently, graphene has been making the mainstream news; the European Commission has pledged €1 billion (yes, that’s billion with a b) to commercialize the fabrication and applications of graphene.

Graphene Production

Graphene was first isolated in its planar form using adhesive tape in 2004. Since then, the race has been on to produce graphene in large quantities using economical fabrication techniques. A more popular method nowadays is to perform epitaxial growth on silicon carbide by heating it to high temperatures at very low pressures. Many other techniques exist, all with their own advantages and shortcomings.

Material Properties of Graphene

Graphene has a set of remarkable material properties: it is roughly 200 times stronger than steel, has a higher electrical and thermal conductivity than copper, and weighs less than 1 milligram per square meter. The high electrical and thermal conductivity can be attributed to the unusually high room temperature electron mobility of 15,000 cm2/(V-s). Researchers are already using COMSOL to study applications that can exploit these unusual properties. In this article the author investigates plasmon modes in graphene rings. This is accomplished by combining Ohm’s law, Gauss’s law, and the continuity equation to produce an integral equation for the electric potential. COMSOL Multiphysics is used to compute the eigenfrequencies of the symmetric and antisymmetric modes for different ring configurations.

COMSOL Multiphysics is the perfect tool to investigate applications of the unusual properties of graphene, since it allows arbitrary expressions and functions to be entered for any material properties required by the model. Check our blog regularly for more upcoming entries on the unusual properties of graphene and how COMSOL can be used to study this new and exciting material!